1. Field of the Invention
The present invention relates to a device for measuring three dimensional shape.
2. Background Art
Generally, when electronic components are mounted on a printed board, firstly cream solder is printed at certain positions on the electrode pattern. Thereafter, the electronic components are temporarily fixed to the printed board by use of viscosity of the cream solder. Thereafter, the aforementioned printed board is conveyed to a reflow furnace, the printed board is subjected to a certain reflow step, and soldering is performed. In recent years, inspection of the printed state of the cream solder has been required at a stage prior the printed board being conveyed to the reflow furnace. A device for measuring three dimensional shape is used during this inspection.
In recent years there have been various proposals for devices for measuring three dimensional shape by the use of light (so-called contact-free devices for measuring three dimensional shape). For example, technology has been proposed relating to devices for measuring three dimensional shape using the phase shift method.
A device for measuring three dimensional shape using this phase shift method uses an irradiation unit composed of a light source emitting a certain light and a grating for transformation of this light from the light source to a light pattern having a sinusoidal wave pattern (stripe-shaped pattern) to irradiate the light pattern on the object to be measured (i.e., a printed board in this case). Then, a point on the board is observed using an imaging unit disposed directly above the board. A charge-coupled device (“CCD”) camera or the like composed of a lens, an imaging element, or the like is used as the imaging unit. In this case, the intensity I of light at a point P on the image plane is given by the below listed formula:I=e+f×cos φ
(within the formula, e=non-modulated light noise (offset component), f=sine wave contrast (reflectivity), and φ=phase imparted by roughness of the object).
Here, due to movement or switching control of the aforementioned grating, the phase of the light pattern is changed, for example, in 4 stages as φ+0, φ+π/2, φ+π, and φ+3π/2. Images of the corresponding intensity distributions (I0, I1, I2, and I3, respectively) are read, and the modulated component α is found based on the below listed formula.α=arctan{(I3−I1)/(I0−I2)}
Using this modulated component α, the three dimensional coordinates (x, y, z) of the point P on the object to be measured, such as a cream solder or the like of a printed board, are found, and these coordinates are used to measure three dimensional shape (particularly height) of the object to be measured.
However, when using only a single aforementioned irradiation unit, shadowed parts may occur where the light pattern is irradiated on the object to be measured (measurement subject) by only one of the aforementioned irradiation units. Thus, there is concern that accurate measurement of such shadowed parts may not be possible.
In consideration of such circumstances, technology is being proposed for the performance of measurement by irradiation of light patterns from two directions in order to improve measurement accuracy or the like.
Previously, in this case, while a grating of a first irradiation unit has been shifted sequentially (or switched), an entire set of image data (e.g., 4 images of image data) has been imaged within a certain measurement subject range (image range) under illumination by a first light pattern having multiply shifted phases. Thereafter, while sequentially shifting or the like a grating of a second irradiation unit, an entire single set of image data is imaged within the aforementioned measurement subject range under illumination by a second light pattern having multiply shifted phases.
In contrast, in recent years, the second light pattern from the second irradiation unit is irradiated, and imaging is performed, while the grating of the first irradiation unit is being shifted or the like, and on the other hand, the first light pattern from the first irradiation unit is irradiated and, imaging is performed, while the grating of the second irradiation unit is being shifted or the like, so that irradiation and imaging are alternatingly repeated so that the measurement time can be shortened (for example, see the proposed technology of Patent Document 1).
[Prior Art Documents]
[Patent Document 1] Japanese Unexamined Laid-open Patent Application No. 2010-276607
However, imaging by a camera or the like is normally performed using a relatively short time interval (e.g., 2 millisecond (“msec”)) under strong illumination in order to reduce the effect of mechanical vibration.
On the other hand, shifting of the grating at the irradiation unit is performed over a relatively long time interval (e.g., 20 msec) in order to avoid vibration or the like. Although a liquid crystal shutter or the like may be used as the grating, switching control of a liquid crystal shutter also requires a relatively long time interval such as that mentioned above.
Thus, according to the configuration of the aforementioned Patent Document 1, for example, assuming a total of 8 imaging operations (4 images each for each light pattern) of a certain measurement subject using two light patterns, assuming that the time interval required for a respective image is 2 msec, and assuming that the required time interval per shifting of the grating is 20 msec, then, as shown in FIG. 6, a relatively long measurement time becomes required until the completion of all processing for the certain measurement subject range (i.e., (first grating shift time interval of 20 msec+second grating shift time interval of 20 msec)×4 times=a total of 160 msec).
Furthermore, if several measurement subject ranges are set on a single printed board, then, the time interval required for measurement of this single printed board becomes several-fold longer. There is thus a need for reduction of the measurement time interval.
The aforementioned problem is not necessarily limited to the measurement of height of a cream solder or the like printed on a printed board, and the aforementioned problem is inherent to the general field of devices for measurement of three dimensional shape.
In consideration of the aforementioned circumstances, one or more embodiments of the present invention provide a device for measurement of three dimensional shape that is capable of improvement of measurement precision and reduction of the measurement time interval when performing three dimensional measurement using the phase shift method.